Underground or submerged metal structures such as well casings and pipelines need a means of protection against soil-side corrosion. Cathodic protection (CP) is a well-known electrochemical method that has proved to be an effective method of mitigating external corrosion on well casings and pipelines.
Cathodic protection for well casings and long pipelines is typically applied in an impressed current configuration, e.g., from an external DC power supply. Impressed current cathodic protection involves the introduction of a conductive material (typically cast iron rods) buried in the ground and electrically connected to the positive (anode) terminal of an external DC power supply. The negative (cathode) terminal of the power supply is connected to the structure to be cathodically protected.
In a perfect cathodic protection system, only the anode's surface corrodes. The transfer of electrical current by a DC power supply through the electrolyte from the anode to the cathode results in corrosion of the anode and overcomes the natural corrosion currents on the cathode, e.g., a pipeline or well casing in this application. Consequently, the anodes require periodic replacement, but the structure of the well casing or pipeline operates for its required life without need for replacement due to corrosion.
FIG. 1 illustrates the setup of a conventional impressed current CP system 10 for a well casing 12. As shown therein, an anode bed 14 is buried in the ground and a cathodic protection DC power supply 16 is connected to the anode bed 14 by a positive cable 18, and to the well casing by a second negative cable 20 at the well head 22 near the ground surface. The cathodic protection DC power supply must provide a sufficient amount of cathodic protection current to protect the portion 24 of the well casing 12 passing through the corrosive zone 26.
Wireline logging tools are commonly used to measure axial current in well casings. The axial currents can be caused by natural corrosion current, by cathodic protection current, or by a combination of both. The measurements of axial currents in a well casing are used to identify anode and cathodic zones and, with cathodic protection applied are used to determine the amount of cathodic protection current required from the DC power supply to offset the natural corrosion currents in the anodic zones.
A common practice in the prior art has been to conduct several logs in a problematic field to generalize the area of corrosion and the amount of cathodic protection required from the DC power supply. The assumption that all wells in a given area are sufficiently similar and that all require the same cathodic protection current is a practical necessity due to the cost and impracticality of conducting an axial current log on every well.
The corrosive areas may have been defined through failure histories, or possibly through wire line logging.
It is an object of the present invention to provide a cost effective and practical alternative for determining the amount of cathodic protection required to protect a new well in a field where the corrosive formations have been reasonably well defined.
Another object of the invention is to provide a permanent installation that will provide a reliable method of monitoring the cathodic protection requirements.
Yet another object of the invention is to provide a method and apparatus for permanent in-situ monitoring of corrosion and cathodic protection current.